Multilayer ceramic substrate and manufacturing thereof

ABSTRACT

A multilayer ceramic substrate includes stacked ceramic layers, and external electrodes including first conductive layers penetrating through one region of an outermost layer of the stacked ceramic layers to thereby be embedded therein, and second and third conductive layers sequentially stacked on the first conductive layers. Each of the first and second conductive layers is formed of a ceramic powder and a metal powder.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. Ser. No. 14/887,108filed Oct. 19, 2015, which claims the priority and benefit of KoreanPatent Application No. 10-2014-0169573 filed on Dec. 1, 2014, with theKorean Intellectual Property Office. The disclosure of each isincorporated herein by reference in entirety.

BACKGROUND

The present disclosure relates to a multilayer ceramic substrate andmanufacturing thereof.

To meet growing demand for miniaturization of electronics, it isrequired that ceramic packages have stable characteristics in highfrequency bands, allowing for the manufacturing of light, thin, andsmall electronic components such as an RF chip, an IC chip, a sensorchip, an X-Tal LED, or the like, from which heat may be easilydissipated, able to be easily surface-mounted, and allowing forincreased product reliability.

To form a ceramic package as described above, a package havingappropriately formed internal patterns, a sufficient electrode materialfilling rate in forming via electrodes, decreased warpage deformation,and a constant pattern interval has been required.

Particularly, adhesive force between a ceramic substrate and an externalelectrode pattern is significantly important for the securing ofreliability of the ceramic package.

To this end, at the time of sintering heterogeneous materials such as aceramic substrate, electrodes for forming an external pattern, andelectrodes for forming vias, or the like, matching of shrinkage behaviorand interfacial coupling force should be secured. In addition, thestructure of electrodes and a compositional design of the ceramicsubstrate are also important.

SUMMARY

An aspect of the present disclosure may provide a multilayer ceramicsubstrate having increased adhesive force between a ceramic layer andexternal electrodes.

An aspect of the present disclosure may provide a multilayer ceramicsubstrate to prevent decrease of adhesive force between ceramic layersand external electrodes from being deteriorated due to mismatching ofheterogeneous materials at the time of manufacturing a multilayerceramic substrate by co-firing the ceramic layers and the externalelectrodes.

According to an aspect of the present disclosure, a multilayer ceramicsubstrate may include: a ceramic layer including a plurality of ceramicsheets which are stacked; and external electrodes each including a firstconductive layer penetrating through one region of an outermost ceramicsheet of the ceramic layer to thereby be embedded therein, and secondand third conductive layers sequentially stacked on the first conductivelayer. The first and second conductive layers may be formed of a ceramicpowder and a metal powder, whereby deterioration of adhesive strengthbetween the ceramic layer and the external electrode may be prevented.

An interfacial area between the via electrode and the external electrodemay be increased by forming the first conductive layer connected to thevia electrode to have a width greater than that of the via electrode,and a compositional ratio between the ceramic powder and the metalpowder in the first and second conductive layers may be controlled, suchthat deterioration of adhesive strength between the ceramic layer andthe external electrode may be prevented.

Therefore, in the multilayer ceramic substrate according to an exemplaryembodiment of the present disclosure, the interfacial area between thevia electrode and the external electrode may be increased, and the firstand second conductive layers of the external electrode are formed of theceramic powder and the metal powder, such that adhesive strength betweenthe ceramic layer and the external electrode after sintering may beimproved, and a difference in firing shrinkage rates between the ceramiclayer and the external electrode at the time of firing may becontrolled, whereby reliability of the external electrode at the time ofsurface-mounting may be improved.

According to another aspect of the present disclosure, a method formanufacturing a multilayer ceramic substrate may include: filling athrough hole of a first ceramic sheet with a first conductive layer, andsequentially stacking second and third conductive layers on a surface ofthe first ceramic sheet to cover the first conductive layer; forming avia electrode in a through hole of a second ceramic sheet; forming aninternal electrode on a surface of a third ceramic sheet; andsequentially stacking the first through the third ceramic sheets on oneanother, and simultaneously firing the stacked first through thirdceramic sheets, the via electrode, the internal electrode, and the firstthrough third conductive layers. The internal electrode may beelectrically connected to the third conductive layer through the viaelectrode formed in the second ceramic sheet and the first conductivelayer formed in the first ceramic sheet. The first and second conductivelayers may be formed of a ceramic powder and a metal powder.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and advantages of the presentdisclosure will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view of a multilayer ceramic substrateaccording to an exemplary embodiment in the present disclosure; and

FIGS. 2 to 4 are cross-sectional views illustrating a method ofmanufacturing a multilayer ceramic substrate according to an exemplaryembodiment in the present disclosure.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings.

The disclosure may, however, be embodied in many different forms andshould not be construed as being limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of thedisclosure to those skilled in the art.

In the drawings, the shapes and dimensions of elements may beexaggerated for clarity, and the same reference numerals will be usedthroughout to designate the same or like elements.

Hereinafter, a multilayer ceramic substrate and a method ofmanufacturing the same according to exemplary embodiments of the presentdisclosure will be described in detail with reference to FIGS. 1 through4.

FIG. 1 is a cross-sectional view of a multilayer ceramic substrateaccording to an exemplary embodiment in the present disclosure.

As illustrated in FIG. 1, a multilayer ceramic substrate 100 accordingto the present exemplary embodiment may include a ceramic layer 110having a multilayer structure, and external electrodes 140 partiallyembedded in the ceramic layer 110 to a predetermined depth. In thiscase, a plurality of internal electrodes 120 and a plurality of viaelectrodes 130 may be formed within the ceramic layers 110.

The ceramic layer 110, in which a plurality of plate-shaped sheets 110a, 110 b, 110 c, and 110 d are stacked, may be formed by stacking aplurality of ceramic green sheets and compressing and firing the stackedceramic green sheets.

The ceramic layer 110 configured as described above may be formed byco-firing the plurality of ceramic green sheets, the external electrodesincluding conductive layers formed in an outermost ceramic green sheetand exposed to the outside of the outermost ceramic green sheet, theinternal electrodes and the via electrodes formed in the plurality ofceramic green sheets so as to be coupled to each other.

Although the ceramic layer 110 in FIG. 1 is illustrated as having fourceramic sheets 110 a, 110 b, 110 c, and 110 d stacked therein forconvenience of explanation, the number of ceramic sheets constitutingthe ceramic layer 110 may be increased or decreased if necessary.

The ceramic layer 110 may include ceramic powder containing a generaloxide or nitride, or the like, used as a low temperature co-firedceramic (LTCC) material or high temperature co-fired ceramic (HTCC)material.

As an example of the ceramic powder constituting a LTCC ceramic layer110, a composite of alumina (A1 ₂O₃) powder and glass powder may beused. As an example of the ceramic powder constituting a HTCC ceramiclayer 110, alumina Al₂O₃) powder may be used.

The internal electrodes 120, used as internal circuit layers, internalpads, or the like, may be formed on the plurality of ceramic greensheets and then co-fired, together with the ceramic green sheets, tothereby be included in the ceramic layer 110.

The via electrodes 130 may penetrate through each of the ceramic sheetsof the ceramic layer 110 to electrically connect the internal electrodes120 and the external electrodes 140 to each other. The via electrodes130 may be formed to fill via holes penetrating through each of theceramic green sheets with an electrode material, and then may beco-fired together with the ceramic green sheets to thereby be includedin the ceramic layer 110.

The internal electrodes 120 and the via electrodes 130 as describedabove may be formed of metal powder capable of being co-fired togetherwith the ceramic layer 110. For example, silver (Ag), copper (Cu),tungsten (W), molybdenum (Mo), or the like, may be used.

Meanwhile, although internal electrodes 120 formed on two ceramic sheets110 a and 110 b and via electrodes 130 formed in three ceramic sheets110 a, 110 b, and 110 c to connect these internal electrodes 120 areillustrated in FIG. 1, the numbers, positions, shapes, and the like, ofinternal electrodes 120 and via electrodes 130 configuring themultilayer ceramic substrate may be variously changed depending on adesign of the substrate.

The external electrodes 140 may be used as connection terminals, or thelike, for forming electrical connections with an electronic component tobe mounted on the multilayer ceramic substrate 100 or electricalconnection with an external terminal.

External electrodes of a general multilayer ceramic substrate are formedof metal powder, and in general, ceramics and metals have differentcoefficients of thermal expansion (CTE). Therefore, it is known that inthe case of co-firing a ceramic layer and a metal layer, adhesivestrength between the ceramic layer and the metal layer may be decreaseddue to the mismatching caused by differences in shrinkage rates betweenheterogeneous materials at the time of firing and stress inherent in aninterface between the ceramic layer and the metal layer.

However, since co-firing technology has advantages such as a simplifiedprocess of co-firing ceramics and metals, and high integration,lightness, thinness, and miniaturization of components in spite of theabove-mentioned disadvantages, it is difficult to exclude co-firingtechnology at the time of manufacturing a multilayer ceramic substrate.

Therefore, according to the present inventive concept, in order toprevent adhesive strength between heterogeneous materials from beingdeteriorated due to mismatches depending on the co-firing of theheterogeneous materials, the configuration of the external electrodes140 is changed, and a detailed example of an application thereof will bedescribed below.

In order to solve the above-mentioned problem, the external electrode140 according to an exemplary embodiment may include a first conductivelayer 142, a second conductive layer 144, and a third conductive layer146 which are sequentially stacked.

First, among the conductive layers constituting the external electrode140, the first conductive layer 142, a layer contacting the viaelectrode 130, may penetrate through a portion of the outermost sheet110 d of the ceramic layer 110 to thereby be embedded.

The first conductive layer 142 may be formed of a mixture of ceramicpowder and metal powder in order to minimize a difference in CTE betweenthe first conductive layer 142 and the ceramic layer 110 at the time offiring.

In view of improving adhesive strength between the ceramic layer 110 andthe second conductive layer 144 of the external electrode 140 andcontrolling firing shrinkage rates thereof, the first conductive layer142 may contain at least 20 wt % of ceramic powder, and more preferably,a compositional ratio of the ceramic powder and the metal powder may be(20 wt % to 30 wt %): (70 wt % to 80 wt %).

Here, when the content of the ceramic powder in the first conductivelayer 142 is lower than 20 wt %, it may be difficult to controlshrinkage behaviors at the time of firing and secure sufficient adhesivestrength, and when the content of the ceramic powder is higher than 30wt %, conductivity of the external electrode 140 may be deteriorated.

Further, since the first conductive layer 142 is connected to the viaelectrode 130, in order to increase an interfacial area between thefirst conductive layer 142 and the via electrode 130, the firstconductive layer 142 may be formed to have a first width W1 greater thanthat of the via electrode 130. When the interfacial area between thefirst conductive layer 142 and the via electrode 130 is increased,interfacial coupling strength therebetween may be increased.

Next, among the conductive layers constituting the external electrode140, the second conductive layer 144 may protrude externally from anexternal surface of the outermost sheet 110d of the ceramic layer 110 tothereby be formed on the first conductive layer 142. That is, the secondconductive layer 144 may be exposed to the outside of the ceramic layer110.

The second conductive layer 144 may be formed of a mixture of ceramicpowder and metal powder in order to improve adhesive strength with thefirst conductive layer 142 while minimizing differences in CTE betweenthe first and second conductive layers 142 and 144, and the ceramiclayer 110 and the second conductive layer 144 at the time of firing.

In view of improving adhesive strength between the ceramic layer 110 andthe first and third conductive layers 142 and 146 constituting theexternal electrode 140 and controlling the firing shrinkage ratesthereof, the content of the ceramic powder in the second conductivelayer 144 may be between the contents of the ceramic powder of the firstand second conductive layers 142 and 146.

The content of the ceramic powder in the second conductive layer 144 maybe at least 5 wt % or more, and a compositional ratio of the ceramicpowder and the metal powder may be (5 wt % to 10 wt %): (90 wt % to 95wt %).

When the content of the ceramic powder is lower than 5 wt %, the effectof the second conductive layer 144 may be insufficient, and when thecontent thereof is higher than 10 wt %, adhesive strength between thesecond and third conductive layers 144 and 146 may be deteriorated, suchthat conductivity of the external electrode 140 may be deteriorated.

As the ceramic powder of the first and second conductive layers 142 and144, a material known in the art may be used without limitation, and theceramic powder may be formed of a general oxide or nitride, or the like,used as the LTCC or HTCC material.

Preferably, in view of controlling a difference in the firing shrinkagerates between the external electrode and the ceramic layer 110, theceramic powder of the first and second conductive layers 142 and 144 maybe the same as that of the ceramic layer 110, and powder of at least oneof magnesium carbonate (MgCO₃), barium carbonate (BaCO₃), calciumcarbonate (CaCO₃), silica (SiO₂), and the like, may be additionallyadded thereto and mixed therewith.

Powder of magnesium carbonate (MgCO₃), barium carbonate (BaCO₃), calciumcarbonate (CaCO₃), silica (SiO₂), and the like, may be used to controlthe difference in the firing shrinkage rates between the externalelectrode and the ceramic layer 110, and decrease or increase a firingtemperature of the first and second conductive layers 142 and 144.

The metal powder of the first and second conductive layers 142 and 144may be metal powder capable of being co-fired together with the ceramiclayer 110. For example, silver (Ag), copper (Cu), tungsten (W),molybdenum (Mo), or the like, may be used.

The second conductive layer 144 may have a second width W2 equal to orgreater than the first width W1 of the first conductive layer 142. Thatis, W1 and W2 may satisfy W1≦W2. A case in which the second conductivelayer 144 has a width greater than that of the first conductive layer142 is illustrated in FIG. 1.

When the second conductive layer 144 has a width greater than that ofthe first conductive layer 142, since a portion of the second conductivelayer 144 not contacting the first conductive layer 142 is bonded to theceramic layer 110, interfacial coupling strength between the ceramiclayer 110 and the second conductive layer 144 may be improved,advantageous for suppressing a delamination defect in which the secondconductive layer 144 and the ceramic layer 110 are separated from eachother.

Then, among the conductive layers constituting the external electrode140, the third conductive layer 146 may be formed on the secondconductive layer 144. In this case, the third conductive layer 146 maybe formed to cover an upper surface and side surfaces of the secondconductive layer 144 in order to complement conductivity.

In addition, the third conductive layer 146 may be formed of metalpowder in order to prevent conductivity of the external electrode 140from being deteriorated. Here, the metal powder constituting the thirdconductive layer 146 may be metal powder capable of being co-firedtogether with the ceramic layer 110. For example, silver (Ag), copper(Cu), tungsten (W), molybdenum (Mo), or the like, may be used, andparticularly, silver (Ag) may be representatively used.

Due to the configuration as described above, in the external electrode140, the content of the ceramic powder may be increased downwardly fromthe third conductive layer 146 to the first conductive layer 142. Thatis, the conductive layer which is closer to the ceramic layer 110 has anincreased content of the ceramic powder.

The external electrode 140 configured as described above may be composedof the ceramic powder and the metal powder, the difference in the firingshrinkage rates between the external electrode 140 and the ceramic layer110 may be controlled and adhesive strength between the ceramic layer110 and the external electrode 140 may be improved by the first andsecond conductive layers 142 and 144 of which the content of the ceramicpowder is controlled to be increased downwardly.

Further, in the external electrode 140 configured as described above,adhesive strength with the first conductive layer 142 may be furtherimproved by forming the second conductive layer 144 having a widthgreater than that of the first conductive layer 142.

As described above, since the multilayer ceramic substrate according tothe present exemplary embodiment may suppress the external electrode 140from being separated from the ceramic layer 140 due to excellentadhesive strength between the ceramic layer 110 and the externalelectrodes 140, at the time of surface-mounting, reliability of theexternal electrode 140 may be improved.

Further, since the difference in the firing shrinkage rates between theexternal electrode 140 and the ceramic layer 110 may be controlled, thegeneration of warpage may be suppressed, such that a substrate havinguniform flatness may be manufactured.

In addition, the multilayer ceramic substrate 100 configured asdescribed above may control the firing shrinkage rates and allow apattern to be formed precisely, and accordingly, a large-area ceramicsubstrate and package may be manufactured, such that a degree of freedomin design for manufacturing a RF module, a sensor module, and a packagemay be secured.

Although the external electrode 140 of which the first conductive layer142 is embedded in the outermost sheet 110d of the ceramic layer 110 isillustrated in FIG. 1, the external electrode 140 is not necessarilylimited thereto. That is, the external electrode 140 may also be formedon both surfaces of the ceramic layer 110 using uppermost and lowermostlayers of the ceramic layer 110.

In addition, although a case in which the number of each of the firstand second conductive layers 142 and 144 is one is illustrated in FIG. 1for convenience of explanation, the number of each of the first andsecond conductive layers 142 and 144 is not limited thereto. The numberof each of the first and second conductive layers 142 and 144 may be twoor more.

Further, in the present disclosure, adhesive strength between theceramic layer 110 and the external electrode 140 may be improved bycontrolling the compositional ratios of the ceramic powder and the metalpowder in the first and second conductive layers 142 and 144constituting the external electrode 140, and by changing a structure, orthe like, but in a case in which powders of the first to thirdconductive layers 142, 144, and 146 constituting the external electrode140 and the ceramic layer 110 are formed to have the same averageparticle size of several μm or so or average particle sizes within asignificant difference of 5 μm or less, adhesive strength between theceramic layer 110 and the external electrode 140 may be furtherimproved. In this case, the ceramic powder and the metal powder may haveaverage particles sizes of about 0.5 to 2 μm, preferably about 0.8 to1.2 μm.

A method of manufacturing a multilayer ceramic substrate according to anexemplary embodiment will be described below.

The method of manufacturing a multilayer ceramic substrate according tothe present exemplary embodiment will be described with reference to themultilayer ceramic substrate illustrated in FIG. 1, and since materialsof a ceramic layer, internal electrodes, via electrodes, externalelectrodes, and the like, may be the same as those in the previousexemplary embodiment, overlapping descriptions will be omitted.

FIGS. 2 to 4 are cross-sectional views illustrating a method ofmanufacturing a multilayer ceramic substrate according to an exemplaryembodiment in the present disclosure. FIG. 2 is a cross-sectional viewillustrating an operation of forming at least one of internalelectrodes, via electrodes, and external electrodes on a plurality ofceramic green sheets, FIG. 3 is a cross-sectional view illustrating anoperation of stacking the plurality of ceramic green sheets on which atleast one of the internal electrodes, the via electrodes, and theexternal electrodes is formed, and FIG. 4 is a cross-sectional viewillustrating an operation of co-firing a ceramic green sheet multilayerbody.

As illustrated in FIG. 2, in order to manufacture the multilayer ceramicsubstrate according to the present exemplary embodiment, first, aplurality of ceramic green sheets 110 a′, 110 b′, 110 c′, and 110 d′ forconstituting a ceramic layer may be prepared.

In this case, the plurality of ceramic green sheets 110 a′, 110 b′, 110c′, and 110 d′ may be formed to have a thickness of several mm by mixingceramic powder, a binder, and a solvent to prepare slurry, applying theslurry to carrier films using a doctor blade method, or the like, andthen drying the applied slurry.

Then, the via electrodes 130 may be formed by etching regions of theplurality of ceramic green sheets 110 a′, 110 b′, and 110 c′ in whicheach of the via electrodes 130 will be formed using laser drilling,mechanical drilling, or the like, to form via holes (not illustrated)and then, inserting a conductive paste containing metal powder, anorganic binder, and a solvent into the via holes.

Next, the internal electrodes 120 contacting the via electrodes 130 maybe formed by printing a conductive paste containing metal powder, anorganic binder, and a solvent on one surfaces of the plurality ofceramic green sheets 110 a′ and 110 b′.

First conductive layers 142 may be formed in the ceramic green sheet 110d′ by etching a region of the ceramic green sheet 110 d′ in which eachof the first conductive layers 142 will be formed to form a through hole(not illustrated) and then, inserting a conductive paste containingceramic powder, metal powder, an organic binder, and a solvent into thethrough hole. Thereafter, second conductive layers 144 may be formed onthe first conductive layers 142 embedded in the ceramic green sheet 110d′ to have a width equal to or wider than that of the first conductivelayer 142 by printing a conductive paste containing ceramic powder,metal powder, an organic binder, and a solvent thereon using a thickfilm printing method, or the like. Then, third conductive layers 146 maybe formed on the second conductive layers 144 by printing a conductivepaste containing metal powder, an organic binder, and a solvent thereonusing a thick film printing method, or the like. Therefore, the externalelectrode 140 in which the first to third conductive layers 142, 144,and 146 are sequentially stacked may be completed.

Next, as illustrated in FIG. 3, a ceramic green sheet multilayer bodymay be formed by sequentially stacking the plurality of ceramic greensheets 110 a′, 110 b′, 110 c′, and 110 d′.

In this case, the ceramic green sheets 110 a′ and 110 b′ in which theinternal electrodes 120 and the via electrodes 130 are formed may besequentially disposed below, the ceramic green sheets 110 d′ on whichthe external electrodes 140 are formed may be disposed thereon, and theceramic green sheet 110 c′ in which the via electrodes 130 are formedmay be interposed between the ceramic green sheet 110 b′ and the ceramicgreen sheet 110 d′.

Next, as illustrated in FIG. 4, the ceramic green sheet multilayer bodyof FIG. 3 may be fired, thereby manufacturing the multilayer ceramicsubstrate 100.

In this case, the plurality of ceramic green sheets 110 a′, 110 b′, 110c′, and 110 d′ (see FIG. 3), the internal electrodes 120, the viaelectrodes 130, and the external electrodes 140 may be simultaneouslyfired by a firing method, and this method is referred to as a co-firingmethod.

The ceramic green sheets 110 a′, 110 b′, 110 c′, and 110 d′ of FIG. 3may be fired to thereby be bonded to each other using the co-firingmethod, such that the ceramic layer 110 corresponding to the multilayerbody in which the ceramic sheets 110 a, 110 b, 110 c, and 110 d aresequentially stacked from the bottom may be formed, and the internalelectrodes 120, the via electrodes 130, and the first conductive layers142 of the external electrodes 140 may be embedded in the ceramic layer110.

An interface between the ceramic layer 110 and the external electrode140 may become dense by the co-firing method, such that adhesivestrength therebetween may be improved.

The co-firing method may be performed at a low temperature of 800° C. to900° C. or a high temperature of 1500° C. to 1600° C. depending oningredients of the ceramic layer.

The multilayer ceramic substrate 100 according to the present exemplaryembodiment, manufactured to have the above-mentioned configuration bythe method as described above may be a high-strength substrate capableof having excellent adhesive strength between the ceramic layer 110 andthe external electrode 140 and decreased warpage deformation even in thecase that the ceramic layer 110 and the external electrode 140 areformed by co-firing.

The multilayer ceramic substrate 100 configured as described above maycontrol the firing shrinkage rates and allow a pattern to be formedprecisely, and accordingly, a large-area ceramic substrate and packagemay be manufactured, such that a degree of freedom in design formanufacturing a RF module, a sensor module, and a package may besecured.

As set forth above, according to exemplary embodiments in the presentdisclosure, even though the multilayer ceramic substrate is formed bythe co-firing method, adhesive strength between the ceramic layer andthe external electrode may be improved due to the external electrode ofwhich the conductive layers adjacent to the ceramic layer are formed ofthe ceramic powder and the metal powder, and the interfacial areabetween the via electrode and the conductive layer is large, such thatreliability of the external electrode may be excellent.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method for manufacturing a multilayer ceramicsubstrate, the method comprising: filling a through hole of a firstceramic sheet with a first conductive layer, and sequentially stackingsecond and third conductive layers on a surface of the first ceramicsheet to cover the first conductive layer; forming a via electrode in athrough hole of a second ceramic sheet; forming an internal electrode ona surface of a third ceramic sheet; and sequentially stacking the firstthrough the third ceramic sheets on one another, and simultaneouslyfiring the stacked first through third ceramic sheets, the viaelectrode, the internal electrode, and the first through thirdconductive layers, wherein the internal electrode is electricallyconnected to the third conductive layer through the via electrode formedin the second ceramic sheet and the first conductive layer formed in thefirst ceramic sheet, and each of the first and second conductive layersis formed of a ceramic powder and a metal powder.
 2. The method of claim1, wherein a concentration of the ceramic powder in the first conductivelayer is greater than that in the second conductive layer.
 3. The methodof claim 1, wherein a compositional ratio of the ceramic powder and themetal powder in the first conductive layer is (20 to 30) wt % to (70 to80) wt %.
 4. The method of claim 1, wherein a compositional ratio of theceramic powder and the metal powder in the second conductive layer is (5to 10) wt % to (90 to 95) wt %.
 5. The method of claim 1, wherein thefiring is performed at a temperature equal to or higher than 800° C. 6.The method of claim 1, a concentration of the ceramic powder increasesfrom the third conductive layer to the first conductive layer.